I think you may have gotten it the other way around - Buffet doesn't feel confident about measuring/guestimating the value of bitcoins, and so he doesn't consider it a currency. A while ago, i read that Buffet's investment strategy is to find companies/stock that is undervalued, and buy it. That means his mindset is that any investment must be made by first making value judgements. In the case of bitcoins, he must be judging the volatility of bitcoins is way too high, and that investing in it isn't a good return on investment.
Bitcoin is not bullion. Bitcoin is not bank notes. Bitcoin is not fiat currency. Bitcoin is something new.
In this case, when Buffett said that bitcoin is not currency, I think he specifically meant that bitcoin is not fiat currency, or any other type of currency where the method of measuring value is well understood.
The main idea behind value investing, as far as I can tell, is to buy things that are being sold for much less than their intrinsic value. If you can't determine the intrinsic value, then on what basis would you invest?
Not that bitcoin doesn't have value - it's just that for someone that practices value investing like Buffett, there's not enough information to decide to invest.
